1. Field of the Invention
The present invention relates to an evaporation apparatus for forming a thin organic film on an object, an organic material evaporation source for use in such an evaporation apparatus, and a method of manufacturing a thin organic film.
2 . Description of the Related Art
The conventional electronic material technology has primarily focused on inorganic materials including semiconductors. However, recent years have seen more attention drawn to functional thin organic films of organic compounds as electronic materials.
Organic compounds are advantageous for use as electronic materials because they provide more diverse reactions and properties than inorganic materials and can be surface-treated with lower energy than inorganic materials.
Functional thin organic films are used in organic electroluminescence devices, piezoelectric sensors, pyroelectric sensors, electric insulating films, etc. Electroluminescence devices can be used as display panels, and efforts are being made to develop a technique capable of forming a thin organic film uniformly on a large substrate for producing an electroluminescence display device with a large display area.
Conventional thin organic film fabrication processes employ vacuum evaporation apparatus primarily designed for forming thin metal films including thin films of Al and SiO.sub.2, and thin organic films. Evaporation apparatus designed for the fabrication of thin organic films have not yet been developed in the art.
Thin organic film materials have features of their own in contrast to thin inorganic film materials as described below.
Thin organic film materials have high vapor pressures. While metal evaporation sources have an evaporation temperature ranging from 600.degree. C. to 2000.degree. C., thin organic film materials have a lower evaporation temperature ranging from 0.degree. C. (often sub-zero temperatures) to 400.degree. C. Many thin organic film materials tend to be decomposed in a temperature range from 20.degree. C. to 400.degree. C. Therefore, it is preferable to effect precise temperature control for evaporating thin organic film materials.
When a thin metal film is to be fabricated, an electron beam evaporation apparatus is used to apply an electron beam to a metal evaporation source. However, if an electron beam is applied to a thin organic film material, the thin organic film material will be decomposed because the energy of the electron beam is too high for the thin organic film material.
Some thin organic film materials are powdery in nature. Generally, powdery materials have poor thermal conductivity. When a powdery material is heated in a vacuum, its temperature cannot easily be raised or lowered due to the heat insulating effect of the vacuum, and the actual temperature of the powdery material may be delayed with respect to a target temperature at which the powdery material is to be controlled.
Once the temperature of a powdery evaporation source is raised, the powdery evaporation source cannot quickly be cooled by radiation only. Therefore, the material evaporation from the powdery evaporation source is not finished immediately when the heating of the powdery evaporation source is stopped. The material evaporation cannot thus be controlled sharply.
Inasmuch as thin organic film materials have high vapor pressures, absorbs on the wall of a vacuum chamber at a low temperature are likely to be released (reevaporated) as the temperature of the vacuum chamber rises. If such released particles find their way into a thin organic film formed on an object, then they tend to degrade characteristics of the thin organic film.
Many thin organic film materials are capable of easily absorbing moisture. Some of them have their properties modified when they absorb moisture. If moisture is trapped into a multilayer thin organic film as it is formed, then properties of the interlayer boundaries are modified. Such property modifications are liable to result in defects in the final performance of functional devices including electroluminescence devices, piezoelectric sensors, and pyroelectric sensors.
Metal evaporation sources exhibits directivity upon evaporation. The movement of vapor from metal evaporation sources is substantially straight therefrom according to the cosine law. The movement of some thin organic film materials vapor, however, is curved like the direction of particle motion due to diffusion.
For forming an evaporated polymeric film, it is necessary that the composition ratio of two thin organic film materials to be evaporated at the same time be in accordance with a stoichiometric ratio. If the composition ratio differs from a stoichiometric ratio, then a fabricated piezoelectric or pyroelectric device, for example, will lose its functions or suffer function degradation. Film growth speeds need to be precisely controlled for equalizing composition ratio to a stoichiometric ratio.
As described above, thin organic film materials have many properties which make themselves difficult to handle with ease.
FIGS. 8A through 8E of the accompanying drawings show various conventional evaporation sources. These illustrated evaporation sources, however, are not suitable for use with thin organic film materials because of the above properties of thin organic film materials and demanded properties of thin organic films.
FIG. 8A shows a direct resistance heating evaporation source including an evaporation source container 101 of metal which is heated by an electric current passing directly therethrough for evaporating a thin film material.
The direct resistance heating evaporation source provides excellent temperature stability in a temperature range in which metals are melted. However, it has poor temperature stability and controllability in a temperature range in which thin organic film materials are evaporated, with the result that an organic compound vapor (the vapor of a thin organic film material) will be produced at an unstable rate.
Some thin organic film materials have an ability to corrode or react with metals. Those thin organic film materials cannot be used with the evaporation source container 101 which is made of metal.
FIG. 8B shows a conical-basket evaporation source having an evaporation source container 111 and a resistance heater 112 disposed around the evaporation source container 111. When the resistance heater 112 is energized, it indirectly heats a thin film material in the evaporation source container 111 to evaporate the thin film material.
FIG. 8C shows a Knudsen-cell evaporation source having an evaporation source container 121 and a resistance heater 122 disposed around the evaporation source container 121. When the resistance heater 122 is energized, it indirectly heats a thin film material in the evaporation source container 121 to evaporate the thin film material.
Each of the evaporation sources shown in FIGS. 8B and 8C provides excellent temperature stability in a temperature range in which metals are melted. However, it has poor temperature stability and controllability in a temperature range in which thin organic film materials produce vapor, with the result that an organic compound vapor (the vapor of a thin organic film material) will be produced at an unstable rate.
The resistance heater 112 usually comprises a bare metal wire. The movement of thin organic film materials vapor is more likely to be curved than that of thin inorganic film materials. If a thin organic film material contains a metal chelate or the like, it may develop a short circuit between turns of the resistance heater 112.
Because the Knudsen-cell evaporation source is of a complex structure, it cannot easily be cleaned, and the thin film material cannot fully be removed from the Knudsen-cell evaporation source after an evaporation process. Therefore, when the thin film material in the evaporation source container 121 is replaced with another thin film material, the new thin film material may possibly be contaminated with a residue of the previous thin film material.
FIG. 8D shows a lamp-heater type evaporation source comprising an evaporation source container 131 made of a light-transmissive material such as quartz and an infrared lamp 133 disposed above the evaporation source container 131. The infrared lamp 133 applies radiant heat to the evaporation source container 131 to evaporate a thin film material in the evaporation source container 131.
The lamp-heater type evaporation source has excellent temperature controllability at low temperatures. However, since the evaporation source container 131 has a large specific heat capacity, there tends to be developed a difference between the a target temperature at which the thin film material is to be controlled and an actual temperature of the thin film material. If the temperature of the thin film material is measured for being controlled, a temperature overshooting tends to occur, decomposing a thin organic film material placed in the evaporation source container 131.
The evaporation source container 131 needs to be made of a transparent material such as quartz or the like in order to allow radiant heat emitted from the infrared lamp 133. However, the transparent material is apt to be damaged when it is cleaned or replaced.
After the evaporation source container 131 has been used for a long period of time, it is irregularly fogged and transmits different infrared intensities at different positions. Such different infrared intensities cause a thin organic film material, which has poor thermal conductivity, placed in the evaporation source container 131 to be overheated locally.
Some thin organic film materials are modified by light at a certain wavelength. Such thin organic film materials cannot be evaporated by the lamp-heater evaporation source shown in FIG. 8D.
FIG. 8E shows an electron beam gun evaporation source which applies an electron beam 145 to a thin film material to evaporate the thin film material. The electron beam gun evaporation source shown in FIG. 8E, however, cannot be used to evaporate thin organic film materials because the electron beam 145 decomposes thin organic film materials when applied to them.
As described above, the conventional evaporation sources shown in FIGS. 8A through 8E will suffer various problems if applied to the evaporation of thin organic film materials. Particularly, the decomposition of thin organic film materials due to a temperature overshooting and the difficulty in heating thin organic film materials are problems that have not been experienced with thin inorganic film materials, and should be alleviated.